Turbojet, turbofan, and rocket engines of current design are generally started using spark discharge igniters that protrude into the engine's combustion chamber, or into a precombustion chamber and ignition flame tube, as in the Space Shuttle main rocket engine. A commonly used igniter type employs a high voltage source (several tens of kilovolts) which is repeatedly discharged across a gap in a manner similar to ordinary automobile spark plugs. One particular design of this type is called a cavity type plasma jet igniter. The spark plugs used with this type system have a small cavity integrated into the region where the electrodes are located. When fired, the arc discharge ejects or puffs-out a heated volume of gas through an orifice in the cavity into the fuel/air mixture, the heated volume including a plasma filament.
Variations of this type of plug, in combination with surface discharge spark initiation features, have become widely used as combustor igniters in turbojet engines, and have seen some application in internal combustion piston engines. It must be recognized, however, that these igniters do not and cannot sustain combustion, they only ignite the flowing fuel-air mixture for the duration of each plasma pulse. As a consequence, other means must be provided to maintain a continuous flame within the engine during operation (generally called “stabilization” of the flame).
A key advantage of cavity type plasma jet spark devices over ordinary sparkplugs is that they launch the short-duration, spark-type, plasma filament away from the combustor walls which could otherwise cool or quench the ignition kernel. By launching it into the combustor, the plasma more readily reaches a region containing a combustible mixture. The fuel/air mixture in a turbojet or rocket combustor region near the walls could be in a less than optimum condition for ignition due to cold surroundings and boundary layer mixing limitations.
On the other hand, cavity type plasma jet spark devices have significant disadvantages in aircraft jet engine applications. In order to deliver the appreciable level of energy necessary to induce ignition, they must operate intermittently. Normally, only about 1 to 2 joules of the 12 to 14 joules developed in a high energy aircraft system reaches the combustion kernel region. Commonly available systems are limited to about 100 to 300 discharges per minute if minimally adequate energy is to be transferred. This is a severe disadvantage during a landing approach or under bad weather flight conditions, when assurance of engine ignition is critical. It is also a disadvantage when attempting to relight a flamed out engine at high altitudes.
Another type of igniter system employs a lower voltage source, perhaps 2 to 5 kilovolts, with a storage means capable of storing 10 to 12 joules of energy. This type of igniter employs a sealed barrier gap switch to hold-off the discharge until a relatively large amount of energy has been built up in the system. The barrier switch fires at a predetermined voltage allowing a discharge across the air gap to proceed. The plugs in this system generally consist of inner and outer concentric electrodes spaced by a ceramic high temperature insulator. The insulator may be coated with a semiconductor material which facilitates ionization in the discharge gap, thus permitting the spark discharge to initiate at the relatively low 2-5 kilovolt voltage. The discharge starts along the surface and builds to a typical ignition spark.
Whichever spark discharge design is used, plug technology is extremely sensitive to excess quantities of fuel (rich mixtures) which can be developed in emergency or inclement weather relight situations. Under these conditions, the discharge can be very substantially reduced and its plasma jet ejection capability almost eliminated.
There have been a number of plasma plume devices built in the past, mostly for use as torches. These devices have not generally been thought suitable for use as engine igniters because they have not been self-starting, requiring some sort of triggering spark to initiate the plasma.
A prior patent, U.S. Pat. No. 5,565,118, of which the present inventor was a co-inventor, discloses a plasma plume igniter driven by a magnetron as is the present invention. One essential difference between the '118 patent and the present invention is that the magnetron in the '118 patent is coupled to its load through a waveguide, as is common with magnetron driven equipment. In the present invention, no waveguide is used; the magnetron is directly connected to a coax transmission line, at the end of which the plasma plume is generated. The discussion in the '118 patent relating to the coax transmission line and the plasma plume generation, as well as other portions of the disclosure, is relevant to the present invention; hence the disclosures of the '118 patent are incorporated herein by reference.